Fig. 4

Co-localized vasa-1, pl10, piwi, nanos-1, and nanos-2 expression in palpons suggests absence of stem cells in mature bodies and spatial restricted pools of i-cells. a–f Gene expression in mature palpons is restricted to the palpacle base and structures at the base of the zooid. Palpons were stained for pl10 (a), piwi (b), nanos-1 (c), vasa-1 (d–e), and nanos-2 (f). e Close-up of the boxed region in d. vasa-1 expression is restricted to the proximal end of the palpacle base, developing bracts, and young female gonophores. f nanos-2 transcript in the basigaster region and the palpacle base. Anterior is to the left. g Semi-thin longitudinal section of the palpon base, stained with toluidine blue, reveals interstitial cells in the palpacle base and nematoblasts (nb) in the basigaster region. Anterior is to the left. h Palpon buds at the anterior end of a cormidium (black arrows) with vasa-1 expression. The sphincter region marks the posterior end of the preceding cormidium. At the site of a sphincter the hollow stem can be constricted. Anterior is to the right. i Close-up of an early palpon cluster bud (arrowhead). j Close-up of a later developmental stage of a palpon cluster with the palpon bud visible in the center and further buds laterally. ba basigaster, bb bract bud, ect ectoderm, end endoderm, fg female gonodendron, ic interstitial cell, m mesoglea, nb nematoblast, p palpon, pb palpon bud, pba palpacle base, sp sphincter, sst siphosomal stem